Xigang Yuan

Computational fluid-dynamics and experimental verification of two-phase two-dimensional flow on a sieve column tray

A modified two-fluid theoretical model is presented to describe the two-dimensional liquid phase flow on a sieve tray with consideration of the resistance created by uprising vapour, and solved numerically by the method of controlled volume. A sieve tray simulator is used for the experimental verification in which the local two-dimensional velocities of the liquid phase are measured by an improved technique using the hot film anemometer. The experimental results show that the liquid phase flow on a tray is substantially two-dimensional if the measurement is made more than 10 mm above from the tray floor, below which the flow is found to be three-dimensional. A comparison between the theoretical computations using the present model, the experimental results produced by the authors and those from the literature indicates that the proposed model gives a good prediction for the two-dimensional liquid phase flow on a tray under normal operation. Factors affecting the liquid phase circulating flow are discussed.

Characterization of Rayleigh Convection in Interfacial Mass Transfer by Lattice Boltzmann Simulation and Experimental Verification

Abstract Concentration gradient induced Rayleigh convection can influence effectively interfacial mass transfer processes, but the convection phenomena are known as mesoscopic and complex. In order to investigate this phenomenon, a two-equation Lattice Boltzmann Method (LBM) is proposed to simulate the velocity and the concentration distributions of Rayleigh convection generated in the CO 2 absorption into ethanol liquid. The simulated results on velocity distributions are experimentally verified by PIV (particle image velocimetry technique) measurements. In order to simplify the analysis, the convection in the simulation as well as in the experiment, the Rayleigh convection was manipulated into a single down flow pattern. The simulated results show that the concentration contours agree qualitatively with the schlieren images in the literature. The experimental and simulated results show that the Rayleigh convection under investigation is dominated by the flow in the downward direction and impels exchange of the liquid between the interfacial vicinity and the liquid bulk promoting the renewal of interfacial liquid, and hence enhances mass transfer. The comparison between the simulated and experimental results demonstrated that the proposed LBM is a promising alternative for simulating mass transfer induced Rayleigh convection.

Simulation based approach to optimal design of dividing wall column using random search method

Abstract A systematic optimization method based on combination of radial basis function neural network (RBF-NN) and genetic algorithm (GA) is developed for optimal design of DWC. The RBF-NN is built by a series of rigorous simulations of DWC for different sets of stage numbers with available simulation software. Then, GA is applied to optimize the stage numbers of the DWC with an objective (cost) function evaluated with the results obtained by the RBF-NN. As a simulation based approach, the proposed method shows its strength in finding the optimal solution by just evaluating small portion of the possible combinations of the stage numbers, and thus can be known as a promising method for optimal design of DWC. Three case studies were solved to detect the optimal structure and their sensitivity to operational conditions was analyzed. The results were shown encouraging compared with those found in the literature.

Thermodynamic analysis of non-isothermal mixing's influence on the energy target of water-using networks

Abstract In this paper, a thermodynamic analysis of non-isothermal mixings influence on the energy target of water-using network is presented. Firstly, water streams in the network are divided into two categories, and then based on the classification all the non-isothermal mixing patterns between two streams are defined. Through thermodynamic analysis from energy composite curve of hot and cold water streams, the influences of non-isothermal mixing on the energy target of water-using network are explored and some mixing rules are obtained, which can be used to simplify the heat exchanger network of a given water-using system and improve the systems energy performance through identifying the beneficial non-isothermal mixings. The applicability of these rules is illustrated by an example in the paper and the result is very encouraging.

A New Model for the Simulation of Distillation Column

A new computational mass transfer model is proposed for simulating the distillation process by solving the fluctuating mass flux for the closure of turbulent mass transfer equation in order to obtain the concentration profile and the separation efficiency of distillation column. The feather of the proposed model is to abandon the conventional way of introducing the turbulent mass transfer diffusivity (dispersion coefficient) to the turbulent mass transfer equation. To verify the validity of the proposed model, a commercial scale packed column and a sieve tray column were simulated and compared with published experimental data. The simulated results were satisfactorily confirmed in both concentration distribution and separation efficiency.

Numerical Investigation on Effect of Vapor Split Ratio to Performance and Operability for Dividing Wall Column

Operability problem of dividing wall column (DWC) raised by vapor split was investigated by numerically analyzing four cases defined by different compositions of a three-component mixture. DWCs were firstly designed for each case by optimizing the vapor split to the two sides of the dividing wall, and then their feasibilities and total annual costs in operation were evaluated against different vapor split ratios. The analysis on the operability of the DWC for four cases was made based on two scenarios: (1) vapor split is shifted by the vapor resistance difference between the column sections in the two sides of the dividing wall and (2) the feed composition is changed. It was demonstrated that the positioning of the dividing wall and the decision on the vapor split may affect significantly the operability of a DWC.

Prediction of Distillation Column Performance by Computational Mass Transfer Method

Abstract A computational mass transfer model is proposed for predicting the concentration profile and Murphree efficiency of sieve tray distillation column. The proposed model is based on using modified two equations formulation for closing the differential turbulent mass transfer equation with improvement by considering the vapor injected from the sieve hole to be three dimensional. The predicted concentration distributions by using proposed model were checked by experimental work conducted on a sieve tray simulator of 1.2 meters in diameter for desorbing the dissolved oxygen in the feed water by blowing air. The model predictions were confirmed by the experimental measurement. The validation of the proposed model was further tested by comparing the simulated result with the performance of an industrial scale sieve tray distillation column reported by Kunesh et al . for the stripping of toluene from its water solution. The predicted outlet concentration of each tray and the Murphree tray efficiencies under different operating conditions were in agreement with the published data. The simulated turbulent mass transfer diffusivity on each tray was within the range of the experimental result in the same sieve column reported by Cai et al . In addition, the prediction of the influence of sieve tray structure on the tray efficiency by using the proposed model was demonstrated.

Control structure selection for four-product Kaibel column

Dividing wall column configurations have a large savings potential in terms of capital and energy. This paper uses dynamic simulation to investigate three alternative control structures for one of these configurations, namely the Kaibel column. Four components, here selected as methanol, ethanol, n-propanol and n-butanol, are separated into pure products within a single column shell. Control structure 1 (CS1) uses only temperature controllers and is therefore particularly interesting from an industrial point of view. Since the control objective is to control the four product compositions, the two other control structures use also composition controllers. Surprisingly, for composition control, the simple temperature control scheme (CS1) is almost as good at steady-state and much better from a dynamic point of view than the two other more complex control structures.

Optimizing and Scale-Up Strategy of Molecular Distillation for the Purification of Lactic Acid from Fermentation Broth

A purification of lactic acid from a fermentation broth was carried out by molecular distillation following the associated calcium precipitation and solvent extraction. Response surface methodology was adopted to investigate the independent experimental parameters. Under the optimal distilling temperature of 367.1 K and operation pressure of 24.5 Pa, lactic acid purity, and yield can reach 92.39% and 74.09%, respectively, with only a single-pass distillation. A multi-pass distillation procedure was further designed to obtain lactic acid with a higher purity about 95.6%. Given the small scale of the laboratory device, a simplified mathematical strategy for scale-up was introduced to serve a practical production.,

A Two-step Design Method for Shaft Work Targeting on Low-temperature Process

Abstract In low-temperature processes, there are interactions between heat exchanger network (HEN) and refrigeration system. The modification on HEN of the chilling train for increasing energy recovery does not always coordinate with the minimum shaft work consumption of the corresponding refrigeration system. In this paper, a systematic approach for optimizing low-temperature system is presented through mathematical method and exergy analysis. The possibility of “pockets”, which appears as right nose section in the grand composite curve (EGCC) of the process, is first optimized. The EGCC with the pockets cutting down is designed as a separate part. A case study is used to illustrate the application of the approach for a HEN of a chilling train with propylene and ethylene refrigerant system in an ethylene production process.